1. Field of the Invention
The present invention relates to a switch, especially to a semiconductor switch.
2. Description of Related Art
Semiconductor device has been used as switch for a long time. For instance, one can use a semiconductor device as a transmitter/receiver switch (T/R switch). A conventional T/R switch 10 is illustrated in FIG. 1, comprising a NMOS transistor 110 which includes a gate-to-drain parasitic capacitance Cgd and a gate-to-source parasitic capacitance Cgs. During the period of the transistor 110 being turned off (i.e. the T/R switch stays at an off state), the drain of the transistor 110 is coupled with a DC operating voltage VDD through a bias resistor 120 and receives an AC voltage signal VAC, and the gate and source of the transistor 110 is coupled to ground. On the basis of the above, in order to prevent the transistor 110 from being damaged or executing malfunction, in the positive half cycle of the AC voltage signal VAC, the drain-to-gate voltage VDG(total) and the drain-to-source voltage VDS(total) of the transistor 110 should be less than the breakdown voltage VBV of the transistor 110; in the negative half cycle of the AC voltage signal VAC, the gate-to-drain voltage VGD(total) of the transistor 110 should be less than the threshold voltage Vth of the transistor 110. Accordingly, the above-mentioned voltage limitation can be shown with the following equations:VDG(total)=(VDD+VAC)−0<VBV  (Eq. 1)VDS(total)=(VDD+VAC)−0<VBV  (Eq. 2)VGD(total)=0−(VDD+(−VAC))<Vth  (Eq. 3)From equations 1 and 3 (or equations 2 and 3), one can derive the highest AC voltage signal VAC which the transistor 110 is able to sustain being (VBV+Vth)/2; in the meantime, the drain-to-gate DC voltage drop VDG(dc) is (VBV−Vth)/2.
The above-described transistor bias configuration has at least the following problems: when the transistor 110 is turned off and the breakdown voltage VBV is not high enough (e.g. the breakdown voltage VBV is twice or less of the DC operating voltage VDD), if the peak value of the AC voltage signal VAC is not less than the DC operating voltage VDD, the peak value of the drain voltage of the transistor 110 will be the DC operating voltage VDD plus the peak value of the AC voltage signal VAC, that is to say that the peak value is 2VDD or more. Meanwhile, because the gate of the transistor 110 is grounded, the highest drain-to-gate voltage drop VDG(total) of the transistor 110 could exceed the breakdown voltage VBV of the transistor 110 (i.e. the aforementioned Eq. 1 is no longer satisfied), and therefore the transistor 110 may be damaged or its lifetime may be reduced.